1. Field of the Invention
The present invention relates to a valley-detection technology, particularly to a valley-detection device for quasi-resonance switching and a method using the same.
2. Description of the Related Art
Flyback is one of the most popular topologies in medium and low power applications due to its simplicity. It can implement isolated switching mode power supplies (SMPS) with one switching component and one transformer resulting in low manufacturing cost. However, the switching losses are unavoidable in hard-switching mode.
As shown in FIG. 1 and FIG. 2, a power converter circuit includes a transformer 10 having a primary winding PW and a secondary winding SW and an N-channel metal oxide semiconductor field effect transistor (NMOSFET) 12 connected to the primary winding PW. An input high voltage VH is applied to the primary winding PW. A voltage VG is periodically applied (TON) to the gate of the NMOSFET 12 to control the transfer of power from the primary winding PW to the secondary winding SW. When the NMOSFET 12 is turned on, the energy is stored in the transformer 10. As the NMOSFET 12 is turned off, the stored energy in the transformer 10 is discharged.
A reflected voltage VR is generated when the NMOSFET 12 is turned off. As a result the voltage VDS across the NMOSFET 12 equals the input high voltage VH plus the reflected voltage VR. While the NMOSFET 12 is turned off, a parasitic drain-to-source capacitor inherent in the NMOSFET 12 stores the energy from the voltage VD.
After a discharge period TDS the energy of the transformer 10 is fully discharge and the energy stored in the parasitic drain-to-source capacitor flows back to the input high voltage VH through the primary winding PW of the transformer 10.
The primary winding PW and the parasitic drain-to-source capacitor along with parasitic elements on the secondary winding create a resonant tank with a resonant frequency fR. While resonating, energy flows back and forth between the primary winding PW and the parasitic drain-to-source capacitor.
When the power converter circuit operates in discontinuous conduction mode (DCM) with a fixed frequency, the drain-to-source voltage of the MOSFET 12 is arbitrary. So the capacitive loss of the MOSFET during switch turn-on transition cannot be predicted and minimized.
To overcome the abovementioned problems, the present invention provides a valley-detection device for quasi-resonance switching and a method using the same, so as to solve the afore-mentioned problems of the prior art.